Electrostatic precipitation has been a reliable technology for about 50 years used to abate smoke and to remove particulates from a waste gas stream. However, electrostatic precipitators are not effective in removing sticky particulate matter from air streams, such as the sticky particulate matter commonly in the waste gas stream in processes used by the wood industry and other industries. Wet electrostatic precipitators were developed for this application about 40 years ago. Electrostatic precipitators and wet electrostatic precipitators work on the principle of electrostatic charging of the particulates in the gas stream. Typically, a collector surface, commonly a bank of tubes, is maintained at positive electrical potential and an electrode is located axially in the tube or tubes connected to a source of electric current and maintained at a negative electrical polarity. A gas stream with entrained particulates is directed through the annular space between the positively charged tube and the negatively charged electrode, imparting a negative charge to the particulates and the particulates then drift toward the collection surface, usually the tube. Upon impacting the tube, the particle charge is released. Where the particles are sticky, however, the particles stick to the tube. To avoid fouling of the collection surface, the gas stream is saturated with a liquid mist, principally water, which collects on the internal surface of the tubes to create a continuous flowing film that keeps the particulate matter from fouling the tubes. Where the electrostatic precipitator includes saturating the gas stream with water, the apparatus is referred to as a wet electrostatic precipitator.
An important parameter in the performance of an electrostatic precipitator, particularly including a wet electrostatic precipitator, is the gas flow velocity through the tubes. A high gas velocity does not allow particulates sufficient time for the particles to migrate from the center of the annular space between the electrode and the internal surface of the tube to the internal tube wall. Therefore, a primary goal in the design of an electrostatic precipitator is to maintain an optimum uniform flow through each of the tubes of the tube bank. However, this is often very difficult to achieve in practice. To achieve more uniform flow distribution through the tubes, the prior devices have added various flow distribution devices, including “air straighteners” and perforated plates as described below. Obviously, these flow distribution devices add cost to the equipment and hinder accessibility to certain parts for maintenance and are susceptible to plugging because they are installed in the “dirty” air stream. Thus, an object of this invention is to eliminate flow distribution devices in the gas stream which contains particulates. Further objectives are reduced costs, improved efficiency and reduction in maintenance costs.
FIG. 1 illustrates one current embodiment of a wet electrostatic precipitator including flow distribution devices as described above. The embodiment of the wet electrostatic precipitator 20 shown in FIG. 1 includes an inlet chamber or inlet housing 22 having an inlet duct or tube 24 and a central outlet duct or tube 26 extend into the inlet chamber 22 having an open end 28 which directs the gas stream received by the inlet chamber 22 into a second chamber 30 as described further below. A bank 32 of conductive tubes 31 each have an open inlet end 34 which receives the gas stream from the second chamber 30. Each of the tubes 31 of the tube bank 32 further includes an outlet 36 which directs the stream of gas into an outlet chamber 38. A negatively charged electrode 40 extends axially into each of the tubes 31. In the disclosed embodiment, the electrodes 40 are supported on a high voltage grid 42 in the second chamber 30. A plurality of nozzles 44 intermittently spray liquid, typically water, into the second chamber 30 to wash away accumulated particulate matter from the insides of the tubes and flow distribution devices. In the disclosed embodiment, a plurality of nozzles 46 are also located in the inlet duct 24 to saturate the incoming gas stream.
As set forth above, wet electrostatic precipitators are utilized primarily for removal of sticky particulate matter from a gas stream, such as a waste gas stream from wood processing applications. The waste gas stream enters the inlet chamber 22 from the inlet duct 24. In the disclosed embodiment of the wet electrostatic precipitator 20 shown in FIG. 1, the inlet housing 22 is cylindrical and the outlet duct 26 is also cylindrical and extends axially into the cylindrical inlet housing 22. The inlet duct 24 in this embodiment is tangential to the cylindrical internal surface of the inlet housing 22 and offset from the axis of the outlet tube 26 to create a cyclonic effect of the gas stream within the inlet housing 22. The gas stream then flows through the open end 28 of the outlet tube 26 into the second chamber 30. The electrodes 40 in the conductive tubes 31 then impose a negative charge to the particulates in the gas stream and the particles then migrate to and collect on the positively charged internal surface of the tubes 31. The particulate matter is then washed through the tubes by the water which collects on the internal surfaces of the tubes 31 from the saturated waste gas stream. As will be understood by those skilled in this art, a wet electrostatic precipitator of the type shown in FIG. 1 may be used independently to remove sticky particulate matter from a waste gas stream or used in combination with other pollution abatement equipment, such as a regenerative thermal oxidizer to remove, for example, volatile organic compounds.
As further set forth above, an object of designers of wet electrostatic precipitators is to maintain a uniform flow through each of the tubes 31 of the tube bank 32. To achieve this object, the wet electrostatic precipitators now include various flow distribution devices, as shown in FIG. 1. In the disclosed embodiment, the outlet tube 26 includes a plurality of radial paddles 48, commonly referred to as “air straighteners.” The purpose of the air straighteners 48 is to provide substantially straighten the flow through the outlet tube 26 into the second chamber 30 to promote more uniform distribution of the air stream through the tubes 31. Further, the disclosed embodiment of FIG. 1 includes perforated plates 50 and 52 adjacent to and at the outlet of the outlet tube 26. In this embodiment, the outlet 54 of the outlet tube 26 is frustoconical. In a typical application, the perforated plates 50 and 52 include a plurality of equally spaced holes having a diameter of 3.125 inches on 3.5 inch centers. A third perforated plate 56 is provided in the second chamber opposite the outlet 54 of the outlet tube 26. In a typical application, the third perforated plate includes a plurality of equally spaced holes or perforations having a diameter of 2.625 inches having center spacing of 3.5 inches. As will be understood by those skilled in this art, the purpose of the perforated plates 50, 52 and 56 are to spread the flow of the gas stream received in the second chamber 30 and equalize the flow through the tubes 31 of the tube bank 32. However, the flow distribution devices, particularly including the perforated plates 50, 52 and 56, collect particulate matter requiring frequent cleaning and maintenance and the air straighteners 48 and perforated plates 50, 52 and 56 add to the cost of the wet electrostatic precipitator.
Further, the flow distribution devices presently used, as shown in FIG. 1, do not evenly distribute air flow through the tubes 31 as shown by FIG. 2. FIG. 2 is a graph plotting the volume of flow through the tubes 31 of FIG. 1 in standard cubic feet per minute across the tube bank 32. As shown by FIG. 2, even with the flow distribution devices described above with reference to FIG. 1, the flow through the central tubes is significantly less than the flow through the tubes at the outer periphery of the tube bank. As described above, this maldistribution of air flow through the tubes 31 results in reduced efficiency of the electrostatic precipitator 20, wherein a greater velocity through the tubes does not allow particulate matter enough time to migrate from the center of the annular space between the electrode 40 and the inner surface of the tubes 31 and a very slow velocity reduces the throughput of the electrostatic precipitator. Thus, an object of any electrostatic precipitator is to have substantially uniform flow of the gas stream through the tubes 31 of the tube bank 32. It will also be understood that the flow distribution devices will also hinder the accessibility of certain parts of the electrostatic precipitator for maintenance or replacement.
The electrostatic precipitator of this invention eliminates the requirement for flow distribution devices, including air straighteners 48 and perforated plates 50, 52 and 56, while providing substantially uniform flow through the tubes at an appropriate volume to provide optimal precipitation of the particulates as now described.